Q&A: Venting a Porch Roof

Q.I’m building a
porch with a hip roof and a solid ceiling. Without
soffit vents or roof vents will the roof get hot enough
to shorten the life of the asphalt shingles? Are there
any other reasons to vent a porch roof?

A.Bill Rose, architect and
building researcher at the University of Illinois
in Urbana-Champaign, responds: I would expect
the overall performance of a vented porch roof and
a porch roof without vents to be about the same. I
advise you to look at porch roofs in your area to
see what the local practice is, and to check with
building code inspectors and follow their
direction.

The joint where the top of the porch roof meets
the wall needs to be flashed well, and most of the
vent details for that look to me like they might
leak water under some conditions of rain and wind.
Soffit vents have much less risk of water
entry.

People have strong and differing opinions on
whether roof venting affects shingle life. Research
on the topic has shown that venting will cool
shingles by a bit, but no one really knows whether
it will enhance the shingle service life, by how
much, what the service life really is, or how it is
quantified.